Yes, this is piecing (both spouses agreeing to work on the marriage together, without the influence of a third party (affair partner)). And it's THE hardest part.
It's going to take her several weeks of "hard withdrawal" to get her OM out of her system, and that's assuming she maintains 100% no-contact with him (each new contact would set her withdrawal "clock" back to 00:00). And then total withdrawal -- and her emotional and sexual feelings for you returning -- could take MONTHS. I'm glad to see you're going to use the help of a MC to help you two navigate this next stage, because it's imperative. And preferably one specifically trained in dealing with infidelity. And a good IC for her on top of it, as you said, would help additionally.
I know these things are tough to hear from her, but at least she's talking to you about them. Work on re-building ATTRACTION first, and emotional and sexual intimacy should follow that.
